1. Field of the Invention
The present invention relates to a security system, and more particularly to a security system which can be armed or disarmed by use of a portable transmitter unit.
2. Description of the Related Art
An anti-theft system (also called a "security system") for preventing a vehicle from being stolen operates by sounding a siren or flashing a vehicle headlight when a thief opens the door, gives a considerable impact to the vehicle or opens the trunk when the security system is armed, and also cuts off the starter of the vehicle engine (i.e., de-energizes the engine) or cuts off the supply of fuel to the vehicle so as to prevent operation of the vehicle, thereby preventing the vehicle from being stolen.
In an example of such a security system, initiation (hereinafter called "arming") or termination (hereinafter called "disarming") of the security operation is performed by a portable transmitter remote control unit (hereinafter called the "Remocon" unit). The arming and disarming are effected by pressing either an arming key or a disarming key both provided on the Remocon unit.
In a security system using such a Remocon unit, there is a problematic situation where the system is armed by accidentally operating the Remocon unit by the driver or a passenger while the vehicle is being driven, thereby bringing the security system into an active state. When the system is armed while the vehicle is in motion, a vehicle vibration sensor or a noise sensor in the system immediately detects vehicle vibrations or noise. As a consequence, the siren suddenly sounds, the vehicle headlights are flashed, the starter is cut off or the supply of fuel to the vehicle is cut off to thereby stop the vehicle, causing great danger.
Therefore, a conventional security system operates to avoid receiving all signals such as an arming signal or a disarming signal, which are transmitted from the Remocon unit, when the vehicle ignition switch is turned on.
The conventional security system thus avoids the above danger caused by the accidental operation of the Remocon unit while the vehicle is being driven. However, such a conventional security system has the following shortcomings. When the security system is armed by the accidental operation of the Remocon unit when the ignition switch is not turned on, the security system is thereby armed or disarmed when this is not intended. For example, when the security system is armed by the accidental operation of the Remocon unit when the ignition switch is off, the system makes the judgment that any subsequent movement of the driver or passenger in the vehicle or a sound from an audio apparatus or the like is abnormal, thereby sounding the siren or flashing the vehicle headlights. Thus, the conventional security system does not avoid the problem caused by the above-described accidental operation of the Remocon unit when the ignition switch is off (i.e., the vehicle is parked).
In the above described case a Remocon unit which is inside the vehicle is accidentally operated. On the other hand, when a Remocon unit outside the vehicle is accidentally operated, the driver also does not want to arm the security system. However, the conventional security system cannot avoid such a situation and in response is armed.